106 ^1^ AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



a muscle be cut. This elastic tension is very favorable to the action of the 

 muscle, as it takes up the slack and ensures that at the instant the muscle 

 begins to shorten the effect of the change shall be quickly imparted to the 

 bones which it is its function to move. The extensibility of the muscle is 

 a great protection, lessening the danger of rupture of the muscle-fibres and 

 ligaments, and the injury of joints when the muscles contract suddenly and 

 vigorously, or when they are subjected to sudden strains by external forces. 

 The importance of extensibility and elasticity to muscles which act as antag- 

 onists is evident. When a muscle suddenly contracts against a resisting force 

 such as the inertia of a heavy weight, the energy of contraction, which puts the 

 muscle on the stretch, is temporarily stored in it as elastic force, and as the 

 weight yields to the strain, is given out again ; thus the effect of the contrac- 

 tion force is tempered, the application of the suddenly developed energy being 

 prolonged and softened. 



4. Influences which Affect the Activity and Character of the Con- 

 traction. (a) The Character of the Muscle. Attention has been called to 

 the fact that irritability and conductivity may be different not only in different 

 kinds of muscle-tissue, and in muscles of different animals, but even in similar 

 kinds of muscle-tissue in the different muscles of the same animal ; the same 

 may be said of contractility. Although irritability, conductivity, and contrac- 

 tility are to be regarded as different properties of muscle protoplasm, they are 

 usually found to be developed to a corresponding degree in each muscle. 

 Those forms of muscle which require for their excitation irritants of slow and 

 prolonged action, are found to conduct slowly and to make slow and long- 

 drawn-out contractions, and muscles which are excited by irritants acting 

 rapidly and briefly are noted for the quickness with which they contract 

 and relax. 



Differences in the activity of the contraction process are made evident 

 by the duration of single contractions of different forms of muscle-tissue. 

 The duration of the contraction of the striated muscles of different animals 

 differs greatly, e. g. of the frog ^ second, of the turtle 1 second, of certain 

 insects only ^ second. Even muscles of apparently the same kind in the 



Pectoralis majo 

 Omohyoid/^T^-^Gracilis 



FIG. 38. Records of maximal isotonic contractions of four different muscles from a turtle, each 

 weighted with 30 grams : Pectoralis major ; omohyoid ; gracilis ; palmaris. The dots mark second, and 

 the longer marks seconds (after Cash). 2 



same animal exhibit different degrees of activity. Cash l reports the following 

 differences in the duration of the contractions of different striated muscles of 

 a frog in fractions of a second: Hyoglossus, 0.205; rectus abdominis, 0.170; 

 gastrocnemius, 0.120 ; semimembranosus, 0.108 ; triceps femoris, 0.104. Sim- 

 1 Archiv fur Anatomic und Physiologic, 1830, suppl. Bd., p. 147. 2 Op. cit., p. 157. 



